The present invention relates generally to stents which are implantable or deployable in a vessel or duct within the body of a patient to maintain the lumen of the duct or vessel open, and more particularly to improvements in stent structures, stenting procedures, and processes for making stents.
Stents are expandable vascular and endoluminal prostheses, usually employed to keep a particular site in the blood vessels open and unoccluded, especially in the coronary and femoral arteries, following treatment such as dilatation by balloon catheter angioplasty (i.e., percutaneous transluminal coronary angioplasty, or PTCA). But these devices are also quite useful in other applications as well, such as in other tracts or ducts in the human body where support of the tract or duct wall is required along a preselected target site to maintain the lumen open and unobstructed. Examples of such other applications are the tracheo-bronchial system, the biliary hepatic system, the esophageal bowel system, and the urinary tract system. A vascular stent in particular must be sufficiently dimensionally stable to keep the vessel and lumen open while resisting recoil of its elastic wall that naturally occurs when the site within the vessel or luminal structure has been subjected to outwardly directed forces that are necessary to expand the elastic fibers, compress fatty deposits on the wall, and/or to deploy the stent, and to prevent an acute closure following dissection of the vessel.
To date, various studies have demonstrated that coronary stenting has major clinical benefits. For example, among several recent studies, those designated as STRESS and BENESTENT have indicated that the implantation of coronary stents can serve not only to reduce acute complications following a balloon PTCA intervention in the coronary system, but also to improve the long term outcome. The local recoil of the vessel wall, which is normally a principal reason for restenosis after balloon PTCA, is prevented as a result of the scaffolding and support of the vessel wall provided by the stent. Experience gained from the profusion of coronary stenting has indicated that patients who had not been considered candidates for the procedure are now eligible for such implantation. In this respect, improvements in the design of stents and in the skill of the implanting physician have been factors in extending the suitability of stent implantation beyond merely type A or B.sub.1 lesions (with reference to the American Heart Association (AHA)/American College of Cardiology (ACC) coronary stenosis morphology), to more severely curved, longer and complex lesions.
According to studies recently performed at the Technische Universtat Munchen and the German Heart Center Munchen, the mortality rate with stents in acute infarction decreases to less than 2%, which represents the lowest mortality reported to date with any kind of treatment for acute myocardial infarction. But the prerequisites for stenting continue to increase, especially for emergency interventions which are frequently encountered in contrast to elective procedures, making it imperative that stent design be improved to enable availability for use over a wider range of coronary morphology types.
Several different stent designs have become available for clinical use or experimental evaluation. Among these are the self-expanding mesh type, the coil type with a helical wire configuration typically of 100 to 200 micron (.mu.) thickness, the slotted tube type such as that disclosed in co-pending U.S. patent application Ser. No. 08/599,880, filed Feb. 14, 1996, and its German counterpart application DE 19537872A1, and the multicellular type which is a modification of the slotted tube type with less surface coverage and smaller openings. Each of these stent designs has its individual benefits and limitations. Slotted tube stents provide the greatest support strength and the lowest recoil, which is particularly important for patients with very hard, even calcified, and a higher degree of stenosis. Recoil of less than 5% is experienced with slotted tube stents, whereas coil and multicellular stents typically allow recoil of about 10 to 15%. Consequently, within a short time after a stent of either of the latter types has been implanted in the vessel, the vessel diameter will decrease to some 10 to 15% of what it was when the balloon was fully inflated to deploy the stent.
While slotted tube stents allow the least recoil, they are somewhat less flexible than the other design types, and, to overcome this reduced flexibility, slotted tube stents of increased length generally incorporate a link or a bridge that interconnects two or more members of the basic principal design, such as shown in U.S. Pat. No. 5,449,323 of Pinchasik et al. Although such a design gives the longer stents--say 15 millimeters (mm), 23 mm, or even 30 mm versions--greater flexibility, it sacrifices mechanical strength at the site of the bridge. Coil stents tend to have wider openings and somewhat greater flexibility than the slotted tube stents, but exhibit less rigidity. Examples of various stent types are found in U.S. Pat. Nos. 5,443,498; 4,580,568; B1-4,733,665; 4,739,762; 5,102,417; and 5,195,984; European patents and applications EP 0221570B1; 0606165A1; and 0364787A1; and German patent DE 4334140A1.
It is a principal aim of the present invention to provide an improved stent of flexible but still mechanically supportive design, which is particularly suited for the longer configurations.